This invention relates to a process for stabilizing trioxane, and particularly relates to a process for preserving trioxane without causing oxidation and decomposition of trioxane.
Trioxane is a cyclic trimer of formaldehyde and is used as a raw material for producing oxymethylene polymer or copolymer and for producing industrial chemicals and is an important material from an industrial point of view. However, trioxane is unstable in the presence of oxygen and particularly so when in a melted state. The presence of even a slight amount of oxygen will oxidize and decompose melted trioxane to form formic acid and formaldehyde. Furthermore, when melted trioxane is solidified by cooling, it is known that, due to the presence of a small amount of impurities, a portion of the trioxane is polymerized to form polyoxymethylene. The trioxane used as a raw material for producing oxymethylene polymer or copolymer must be of high purity, the proportion of impurities, such as formic acid and formaldehyde, being very low or nil. Particularly, it is necessary that the formic acid content of the trioxane be less than 20 ppm.
Many prior art attempts have been made to preserve trioxane. Theoretically, oxidation and decomposition of trioxane can be prevented by preserving it in the absence of oxygen, but it is difficult to replace oxygen in the atmosphere, in which trioxane is placed, by an inert gas free from oxygen. It is known that addition of phenolic antioxidants or disulfides to trioxane will preserve it. However, though phenolic antioxidant is effective for preventing the formation of formic acid, it has little effect for preventing the formation of polyoxymethylene in case of solidifying trioxane by cooling it.
When trioxane is solidified by cooling it, the amount of polyoxymethylene formed is as high as several percent on the basis of weight of trioxane. When trioxane having such a high content of polyoxymethylene is melted again, the polyoxymethylene is precipitated in a white cotton-like state or white membrane state. This precipitate adheres to the inner surface of the pump, pipe or tank for storage. This results in clogging of the pipe, etc. and hinders the operation of the apparatus.